Method for Interference Suppression in a Measuring Device

ABSTRACT

The present invention describes a method enabling one to shield a device that measures weak biomagnetic signals from strong magnetic interference fields. The measurement sensors are provided with a feedback compensation loop, the difference signal of which is obtained from the measurement sensors themselves. As the actuator of the feedback function, one or more coils are responsible for eliminating, the external interference fields in the region of the sensors. Difference signals can be generated as a linear combination from the signals of two or more sensors. In the control logic, the SSS method can be used to numerically separate the biomagnetic signal being measured from the signals produced by the sources—compensation coils and interference sources—disposed outside the measurement region. The interference suppression can be enhanced by placing the assembly of sensors and the actuators within a magnetically shielding room.

FIELD OF THE INVENTION

The invention relates to the shielding of a measuring device fromexternal magnetic interferences.

BACKGROUND OF THE INVENTION

A device that measures weak biomagnetic signals is very susceptible tothe influence of the strong magnetic interferences in its operationalenvironment. This is due to the fact that compared to the biomagneticsignals being measured, the interference signals are even ten milliontimes bigger. Furthermore, the implementation of the interferencesuppression is made more difficult because the region to be shieldedfrom magnetic interferences is relatively large, tens of centimetres inits diameter.

To make biomagnetic measurements, several methods for protectingmeasuring devices from interference fields have been developed, whichinterference fields are many times larger than the interesting signals.A straightforward method of shielding is to place a sensitive magneticmeasuring device inside a so-called magnetically shielding room whichsuppresses magnetic fields originating from sources outside the roominto about 100-10,000th part.

In addition to this, to achieve magnetic shielding, it is known to usesensors the geometrical structure of which makes them unsusceptible torather steady magnetic fields originating from distant sources. Magneticsensors of this kind are called gradiometers. Typically, a shieldingfactor of about 100-1,000 against external interferences is obtainedusing them.

Further, the magnetic shielding can be implemented, or it can beimproved, using active systems in which the magnetic interference iseliminated by means of a suitable control system in which theinterference is measured in the vicinity of the region being shielded bymeans of a sensor or sensors; and based on this measurement, theinterference field is compen—sated with current-carrying coils thatproduce by a magnetic field that is opposing with respect to theinterference. Active magnetic shielding can be used either alone orcombined with passive shielding methods such as a magnetic shieldingroom.

In this control system it is possible to use either direct coupling orfeedback. When using direct coupling, the measuring device associatedwith the control system is disposed far from the actuator and from theregion being compensated for inside the coil or coils. In this case, thecontrol system functions simply so that one inputs into the coils acurrent that is proportional to the interference measured by themeasuring device and is of the kind that a compensating field having, asaccurately as possible, the size of the interference is formed in theregion being shielded. With this kind of system it is also possible tocombine a magnetic shielding room.

The performance of a compensation based on direct coupling usually israther limited because the field intensity to be compensated for isdetermined far from the region being shielded. This functions in thecase of one or two stationary interference sources, but when there arethree or more sources, it usually is impossible to find a place for thesensor from which the field produced by all the sources could becorrectly extrapolated for the region being shielded. This kind ofshielding method usually gives a shielding factor of about 3 to 10,depending on the number of interference sources. The method onlyfunctions for interference sources that are disposed clearly fartherfrom the region being shielded than the sensor controlling the controlsystem. The method works worse for interference sources that aredisposed just a little farther than the sensor controlling the controlsystem, and specifically for sources that are disposed nearer than thesensor it does not work at all.

The sensor of the control system can also be introduced inside thecompensation coil assembly near the region in which there is a wish tocompensate for an interference. In that case, it is a question about afeedback control system which works better than a directly coupled onealso for more complicated interferences originating from many differentsources. Publication EP0514027 shows an example of a feedback controlsystem enabling one to minimise the effect of a magnetic interference.With a feedback control system it is also possible to connect a magneticshielding room either so that the compensation coils are disposedoutside the magnetic shielding (U.S. Pat. No. 3,801,877) or inside(EP0396381 or corresponding U.S. Pat. No. 4,963,789).

Publication EP0966689 discloses a magnetic gradiometer which is used tomeasure diverging components of a magnetic field. In particular, theequipment can be used to measure a small changing field irrespective ofthe earth's magnetic field (gradient component of the magnetic field).The equipment includes at least two magnetic detectors, feedback coils;and, moreover, each feedback loop includes an amplifier and anintegrator. At least two detectors have been adjusted to detect themagnetic field in the same direction. The purpose of the feedback coilsis, by imitating the magnetic field produced by the environment, toeliminate the effect of interferences in the total magnetic field to bemeasured using detectors. To mutually balance the detector outputs, thedetector outputs are processed with a signal-processing algorithm. Thetotal energy to be obtained as a sum of the detector outputs isminimised to find out the components of the magnetic field.

In the method as shown in publication EP0966689, a biomagnetic signalbeing measured becomes distorted as a result of active compensationbecause the magnetic detectors of the method are used to detect bothbiomagnetic and interference signals. Publication EP0966689 does notpresent means to correct this distortion.

In biomagnetic applications, the volume on whose region the sensors aredistributed, typically is tens of centimetres in its diameter, that israther large. If there is a wish, in addition, to keep the set ofreference sensors used for the compensation far from the source ofinteresting biological signals like in the prior art—then the volumecontaining sensors is even 50 cm in diameter. The compensation of amagnetic interference, e.g. with the accuracy of a percent (itsreduction into its hundredth part), by using feedback, requires that theset of compensation coils is capable of producing the fieldscorresponding to the geometry of the interference fields with theaccuracy of a percent in this entire volume that contains both themeasurement sensors and the reference sensors that produce thedifference signals of the control system. Only in this situation thecontrol system is correctly informed of the interference to becompensated for, and the interference is compensated for in all themeasurement loops, with high accuracy.

The compensation using coils producing a compensating field is made themore accurate the smaller is the volume being compensated for. For thisreason it would be desirable to place the sensor of a feedbackcompensation system as near as possible the actual sensors of themeasuring device. Previously, one thought that this cannot be donebecause in that case one would also compensate for the signal to bemeasured, as if it were an external interference.

The problem with the prior art is thus the inaccuracy of thecompensation in the region of the entire set of sensors, because theinterference field is measured outside the assembly of sensors. Using aseparate set of reference sensors also makes the equipment toocomplicated.

OBJECTIVE OF THE INVENTION

The objective of the present invention is to present a solution in whichthe feedback information needed for the compensation of theinterferences is obtained from the sensors used for the measurementitself, that is from those sensors that one intends to protect frominterferences. This enables one to reach a very effective interferencesuppression because the interference is measured in that very placewhere it must be eliminated, and the size of the region to becompensated for is as small as possible.

SUMMARY OF THE INVENTION

The present invention discloses a manner deviating from the prior art ofimplementing magnetic shielding using a feedback compensation systemthat does not need a separate reference sensor or sensors giving thedifference signal. In particular, the present invention can be appliedto magnetographic devices (MEG) which are used to measure weakneuromagnetic signals originating from the brain.

In this method, the actual set of measurement sensors is provided withtwo separate feedback loops. The inner feedback loop is responsible forthe feedback of a signal that is small-feature with respect to itsgeometry and originates from an object being monitored, and the outerfeedback loop takes care of the feedback of interference signals thatare bigger with respect to their amplitude and geometry. It can also besaid that the inner feedback loop is a channel of an MEG deviceoperating in a flow-locked state, and the outer feedback loop is afeedback loop that controls the compensating actuator. This enables oneto end up in a situation in which big external interferences only arevisible in the outer feedback loop, and do not eat away the dynamics ofthe inner feedback loop that contains the actual interesting signal. Ina preferred embodiment, one feedback loop includes an amplifier and afeedback resistor.

The sum signal of an interference signal and an interesting signal beingmeasured is thus measured using the sensors of the actual set ofmeasurement sensors, by means of which the difference signal isobtained. Difference signals can be generated as a linear combinationbased on one or more signals measured by the measurement sensor. Thecompensation voltage to be obtained by means of a difference signalproduces a current in the actuator. The actuator typically is a coilthat produces a compensating magnetic field. There can be several ofthese. Compensation voltages can be produced several by means of variouslinear combinations based on the channels of the assembly of sensors,and these voltages can be fed into the different coils of thecompensating actuating system by weighting using suitable weightingfactors. A compensating magnetic field eliminates the interferences tobe detected in an assembly of sensors, enabling one to dependablymeasure the desired biomagnetic signal that is considerably smaller insize. In that case, by means of the interference compensation, thesensors can be made to operate in their dynamic operating range becausethe possible high-amplitude interferences have been compensated for.

The compensating actuators, i.e. typically coils, can be placed near theassembly of sensors, but, however, farther from the biomagnetic signalsource than the sensors. The coils can be secured to a separate frame orsome other solid surface.

The mixing of the big interference signal to be processed in the outerfeedback loop and the compensating signal with the small-amplitudebiomagnetic information included in the inner feedback loop is preventedusing the mathematical SSS method (SSS=Signal Space Separation),described in publication FI20030392 (Taulu S., Kajola M., Simola J.: TheSignal Space Separation method, Biomed. Tech., 48, in press).

In the SSS method, a magnetic field measured using a multi-channel MEGdevice is analysed by examining three volumes of different measurementgeometries. The interesting source is disposed in measurement volume V1;the sensors are disposed in measurement volume V2 outside volume V1. Thesources of magnetic interferences and the compensation coils are outsidethe aforementioned volumes in volume V3. In this examination, the V3 canalso be infinite in volume. In the method, the magnetic field producedby the interesting sources disposed in volume V1 is parametrised involume V2 as a sum of elementary fields, each of them beingirrotational, sourceless and infinite outside volume V1, so that apresentation of the desired accuracy is achieved for the parametrisedmagnetic field in volume V2. Similarly, the sum magnetic field producedby the interference fields and compensation coils disposed in volume V3is parameterised in volume V2 as a sum of elementary fields. The signalvectors of the measuring device corresponding to each elementary fieldare calculated. If a magnetic signal is measured using sensors, thenthereafter, the fields produced from sources disposed in differentvolumes can be separated by calculating the components of the measuredsignal vector in the basis formed by the signal vectors associated withthe elementary fields.

The operation of the interference compensation can be optimised bymodifying the values of the feedback resistors, by increasing the numberof compensation coils and by changing the places of the coils withrespect to the sensors and the measurement object.

In one embodiment of the present invention, the assembly of sensors andthe coils functioning as the actuators can be placed inside amagnetically shielding room. This enables one to improve theinterference suppression.

The present invention is simpler than the prior-art solutions because asthe sensors measuring the level of interference, the measurement sensorsof the biomagnetic signal itself are used. The interference suppressionis also made more effective because the interferences are measured inthe very place where there is a wish to eliminate them. Further, thevolume of the region being compensated for is small in the case of thepresent invention.

As compared to the prior art represented by publication EP0966689, theinvention has the advantage that the signal distortion caused by theactive compensation is corrected by means of the SSS method.

LIST OF FIGURES

FIG. 1 represents equipment measuring a neuromagnetic signal, as a partof which there is actuator equipment that compensates for theinterference;

FIG. 2 illustrates a circuit diagram of feedback for one measurementsensor;

FIG. 3 represents the functional diagram of a control system providedwith two feedback loops; and

FIG. 4 illustrates a generalisation of the compensation method as shownin FIGS. 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION

The substantial principles of the invention are apparent from theaccompanying figures. The total structure of the equipment itself isapparent from FIGS. 1 and 2. FIGS. 3 and 4 are functional diagrams thatmainly describe the proceeding and processing of the signals in theequipment presented. FIG. 1 illustrates a so-called MEG apparatus whichis used to measure a neuromagnetic signal and which has, as a partthereof, a system compensating for the interferences. The deviceconsists of an assembly of sensors 10 (including nine sensors in theexample of the figure) surrounding the head of a person being monitored,of the electronics 11 controlling the operation of the measuring deviceand of the coils 12, 13 used as the actuators of the interferencecompensation system.

Associated with the sensors of each device is a small-sized feedbackcoil 14, by means of which the control electronics 11 runs the sensor ina so-called flow-locked state. This means that the control electronics11 drives current into the feedback coil 14, the field caused by whichsuppresses in the sensor 10 the field entering it and originating from asource disposed in the object being measured 15. The voltage needed toachieve this current, which is thus proportional to the magnetic effectproceeding from the source 15 to the sensor 10, is the measurementsignal given by the channel in question. All the conventional MEGdevices have been implemented using this principle.

The feedback coils 14 are sensor-specific The coils are so small and sopositioned that a field caused by them only produces a effect on thesensor of each coil's own. It can be considered that the sensor 10 andthe feedback coil 14 together form the component functioning as thephysical sensor. In an arrangement such as this, all the measurementchannels naturally react to the magnetic fields originating both fromthe object 15 being measured and from the external interference sources.

To achieve active compensation that shields from external interferences,in the present invention, the measuring device is provided with bigcompensation coils 12, 13, the current fed into which produces amagnetic field in the entire region of the assembly of sensors. Therecan be several compensation coils e.g. six pieces—so that the coils areused to produce compensating fields near the assembly of sensors atleast in three nearly perpendicular directions.

How the operation of the system is controlled by means of theelectronics 11 is apparent in more detail from FIGS. 2, 3 and 4. FIG. 2is a simplified representation illustrating a situation in which e.g.the sensor 10 of FIG. 1 has been fed back via the compensation coil 12.The part within the broken line is a regular MEG channel operating in aflow-locked state, and associated therewith are an amplifier 20, afeedback resistor 21 and a feedback coil 14, which switches to thesensor 10 via mutual inductance M_(f). The part divided by the brokenline can be seen as the actual measurement channel containing, in thepreviously mentioned wider sense, the sensor 10, 14; and the electronicspart 20, 21 disposed within the electronics 11. The voltage U₀ is thesignal of the channel that is proportional to the magnetic flux Φ_(s)visible to the sensor 10. Thus, the magnetic flux comprises the sum ofthe interesting magnetic flux to be measured and of the magnetic fluxproduced by external interferences in the location area of the sensor10.

When this measurement channel is accepted as the channel that gives thedifference signal of the feedback active compensation system, thecontrol electronics is provided with an amplifier 24, a feedbackresistor 25 and a coil 12. The magnetic field is transferred from thecoil 12 to the measuring sensor 10 via the mutual inductance M_(c).

In FIG. 3 there is a functional diagram illustrating this control systemprovided with two feedback loops, for which one can calculate a transferfunction. The inner feedback loop includes an amplification block 30, atransfer function 31 corresponding to the resistor 21 and a transferfunction 32 corresponding to the mutual inductance M_(f). The outerfeedback loop compensating for the interferences includes anamplification block 33, a transfer function 34 corresponding to theresistor 25 and a transfer function 35 corresponding to the mutualinductance M_(c). In the input of the amplifier 30 (the same as theamplifier 20 in FIG. 2), there is summed the magnetic flux from theexternal interference source Φ_(s) and the magnetic flux Φ_(c)compensating for the interferences, as well as the magnetic flux fromthe inner feedback that switches via the coil 14. This summation isperformed in the functional block using summing blocks 36, 37, which donot exist in practice as real components of the system.

The output voltage U₀ of the channel and the voltage U_(c) of the outerfeedback loop will be:

$\begin{matrix}{U_{0} = \frac{G_{1}\Phi_{s}}{1 + \frac{G_{1}M_{f}}{R_{f}} + \frac{G_{1}G_{2}M_{c}}{R_{c}}}} & (1) \\{U_{c} = {G_{2}U_{0}}} & (2)\end{matrix}$

If the outer feedback loop is omitted (G₂=0), there remains aconventional feedback magnetometer—a part which is disposed within abroken line in FIGS. 2 and 3 and the calibration of which is determinedby the transfer function of the inner feedback loop:

$\begin{matrix}{{U_{0} = {\frac{R_{f}}{M_{f}}\Phi_{s}}},\mspace{14mu} {{{when}\mspace{14mu} G_{1}\frac{M_{f}}{R_{f}}}\operatorname{>>}1}} & (3)\end{matrix}$

When the outer feedback loop is introduced, provided with sufficientamplification, in other words when G₂*(M_(c)/R_(c))>>M_(f)/R_(f), then:

$\begin{matrix}{U_{0} = {{0\mspace{14mu} {and}\mspace{14mu} U_{c}} = {\frac{R_{c}}{M_{c}}\Phi_{s}}}} & (4)\end{matrix}$

As a result of introducing the outer feedback loop, the output signal ofthe channel is thus lost, and a current that produces a compensatingmagnetic field appears in the compensation coil 12. When a signal isproduced by an outer interference source and if one has managed to buildthe compensation coil(s) 12 so that it produces in the entire sensorregion a field that is as closely as possibly of the same form as thisexternal interference source, then the same compensation of the outputsignal is performed also for all the other sensors in the assembly,although in their feedback, the outer loop has been omitted (G₂=0). Thisis exactly the shielding effect at which one aims by adding the outerfeedback loop.

As for the control of the outer feedback loop, the same magnetomerchannels are used that are also used to measure the biomagnetic signalbeing monitored, it is obvious that the outer feedback also influencesthis biomagnetic signal. For example, a channel that has an outer loopadded to its feedback (G₂>0) also looses the biomagnetic signal as aresult of this arrangement.

The basic idea of the present invention is included in how thisnon-desired effect can be prevented in a simple manner. Firstly, wethink of a system in which there are no feedback loops activated. Themeasurement channels of a system such as this register both the biginterference signals from externals sources and weak biomagneticsignals. Publication FI20030392 discloses a method (SSS method); TauluS., Kajola M., Simola J.: The Signal Space Separation method, Biomed.Tech., 48, in press) enabling one, in a situation such as this, toseparate from one another, with a high accuracy, the signals that comeoutside and inside the device's measurement region, provided that thearrangement of the device's channels is suitable and their numbersufficient (at least 200).

This numerical method would be enough as such to eliminate the externalinterferences from a measured signal, provided that the interferenceswould remain so small that the dynamic region of none of the measurementchannels of the set of sensors is not exceeded. It is exactly thisexceeding that can be prevented with the compensation method describedin the present invention. As the compensation method is implementedusing coils placed outside the measurement region of the device, theeffect of the compensation current associated with this outer feedbackloop on the signals can be likewise separated, using the SSS method,from a signal originating from the measurement region.

As an example, we refer to a situation in which a shielding method basedon an outer compensation loop would seem to function in a mostimpractical manner. We assume that there are no external interferencesand that the sensor functioning as the difference signal channel onlysees the biomagnetic signal. It reacts to this by feeding to thecompensation coil a current that produces a compensating field thatcompensates for the biomagnetic field at the spot where the sensor islocated. Apparently, the feedback of the outer loop thus functions sothat although there is no external interference, it is exactly theinteresting signal that is lost.

Thus, the biomagnetic signal is driven to zero by activating thecompensating source of the magnetic field, external of the measurementregion. It is exactly the effect of this kind of external source thatcan be numerically separated by the SSS method, resulting in that thereis left in the difference signal channel just the original biomagneticsignal. The SSS method reconstructs in the difference signal channel—andin the rest of the channels as well—signals that would have beendetected in them, if the outer feedback loop was not activated. Thisreconstruction is based on the measurement that is made simultaneouslyfor both the biomagnetic source and the compensation coil by theassembly of magnetometers.

The SSS method naturally functions in the same manner in conjunctionwith interference compensation, that is when the difference signalchannel receives part of its signal from an interference source that isdisposed outside the measurement region or even outside the entire setof compensation coils. In these cases, both the original source and thecompensation coil that activates as a part of the feedback loop aredisposed outside the measurement region; and their portion of thesignals can be eliminated by the SSS method. In this case, thecompensation system only is responsible for modifying the externalinterference so that all the sensors stay in their dynamic area,enabling one to collect the signals needed by the numerical system asinputs.

Because as the interference to be compensated for functions a vectorfield which is not constant in the region of the entire assembly ofsensors, to achieve a sufficiently good compensation, it is usuallynecessary to use a set of compensation coils that can be used to achievevarious fields in direction and form. Specifically the set of coils mustbe able to produce the field forms of the most powerful externalinterferences as accurately as possible, or to be more specific, thecompensating fields of these, in the entire region of the assembly ofsensors. FIG. 4 is a functional diagram illustrating a generalisation ofthe compensation method formed by several compensation coils as shown inFIG. 3.

In FIG. 4, the voltage U_(c) that produces the compensation current hasbeen coupled with two coupling intensities to be selected separately(1/R_(c,j) and 1/R_(c,j+1)) 40, 41 to two different coils 42, 43,respectively. The inductive couplings M_(ji) etc. 44 are determinedbased on the location of the compensation coils 43, 43 and on thelocation and position of the sensors 45 in the assembly of sensors.U_(c) can be coupled to more than one coil. In addition, in FIG. 4, thedifference signal that produces the compensation voltage has been formedas a linear combination from the signals of two different sensors 45. Inthe formation of the linear combination, the weighting coefficientsC_(i,j) 46 are used for the sensors 45; and the terms are summed by thesummer 47. It also possible to use more than two channels for theformation of the linear combination. The necessary transfer function ofthe amplifier of the feedback loop is G₂, 48. Furthermore, it is alsopossible to generate several compensation voltages U_(c),n using for theformation of the difference signal, different linear combinations of thesignals of the assembly of sensors 45 and to feed the voltages to thedifferent coils 42, 43 of the set of compensation coils using optimalweighting coefficients. In the functional diagram as shown in FIG. 4,the sum flow of the compensating magnetic flows visible to the sensor 45is obtained from the outputs of the summers 49. In practice, the summers49 do not exist in the system as real components. The summers 49 areused to describe the total effect of the fields produced by thecompensation coils (the total field is the sum of the sub-fields) foreach sensor.

In this manner there is formed the outer feedback loop that performs theinterference compensation of a multi-channel device, which feedback loopis described by the two matrixes: the cij matrix that describes thestructure of a direct coupling loop and determines the weightingcoefficient of the ith sensor 45 in the jth difference signal, and the1/R_(jk) matrix that describes the feedback loop and determines theweighting coefficient of the jth compensation current in the totalcurrent to be fed to the kth coil 42, 43.

The selection of these two matrixes enables one to optimise theperformance of the interference compensation. The compiling of thedifference signal as a linear combination from several channels that aree.g. disposed on different sides of the assembly of sensors isadvantageous because it improves the accuracy of the difference signaland shortens the effective extrapolation distance over the assembly ofsensors. In a conventional system that uses separate reference sensors,the sensor that gives the difference signal can be disposed even at adistance of 50 cm from the farthest sensor being compensated for in theassembly of sensors, resulting in that the interference intensityevaluated based on the difference signal is inaccurate because of thegeometric reasons. By forming the difference signal from the signals ofthe sensors disposed on different sides of the assembly, theextrapolation range can be shortened to have the size of the radius ofthe assembly, that is to about 12 cm.

By means of the compensation loop, the field forms that were fed backcan, in turn, be customised, to correspond, as accurately as possible,to the geometric form of the biggest external interferences by using asufficiently big number of compensation coils and by determining thecorrect weighting coefficients for the 1/R_(jk) matrix. A typical numberof compensation voltages U_(c,j) and compensation coils is e.g. six,whereby the 1/R_(jk) matrix is 6*6 matrix.

In FIGS. 1-4 there have been used symbols that refer to theimplementation of the feedback that compensates for externalinterferences using analogy electronics. This has been done forillustrative purposes only. In a modern implementation, one uses signalprocessors or real-time computers in which the weighting coefficientmatrixes c_(ij) and 1/R_(jk) and the transfer function G₂ areprogrammed.

The invention is not limited merely to the embodiment examples referredto above; instead many variations are possible within the scope of theinventive idea defined by the claims.

1. A method for shielding an assembly of sensors that measuresbiomagnetic signals from external interferences, the assembly of sensorscomprising at least one sensor, characterised in that the methodcomprises the steps of: providing the assembly of sensors with at leastone magnetic feedback, wherein the difference signal of one feedback isobtained from at least one sensor of the aforementioned assembly ofsensors; producing in the region of the assembly of sensors a magneticfield that compensates for external interferences with at least oneactuator disposed outside the assembly of sensors; and the differencesignal that produces a compensation voltage in the actuator is a linearcombination of the signals of one or more sensors; and using the SSSmethod to separate the biomagnetic useful signal being measured from thesignals (originating from outside the measurement region), which signalsare produced by the actuators and the external interferences.
 2. Themethod as defined in claim 1, characterised in that the method furthercomprises the steps of: forming a difference signal as a linearcombination from the signals of two or more sensors of the assembly ofsensors; and feeding back the difference signal to the actuators, thecompensating magnetic field being formed as a linear combination fromthe magnetic field produced by at least one actuator.
 3. The method asdefined in claim 2, characterised in that the method further comprisesthe step of: selecting the difference signals to be obtained as a linearcombination and the actuators to be used so that as the feedback isswitched on, the external interference signal is minimised.
 4. Themethod as defined in claim 1, characterised in that the method is usedin a magnetoencephalographic device (MEG).
 5. The method as defined inclaim 4, characterised in that the measurement sensor is coupled to twofeedback loops, the first of them being a channel of an MEG device thatoperates in a flow-locked state and the second a feedback loop thatcontrols the compensating actuator.
 6. The method as defined in claim 1,wherein in the SSS method, a magnetic field that has been registeredusing a multi-channel measuring device is analysed in a geometry inwhich the interesting source is disposed in measurement volume V1, thesensors measuring the field or the components thereof outside volume V1in volume V2, and the sources of the magnetic interferences and theactuators outside volume V1 and V2 in volume V3, which can be infinite,characterised in that the method further comprises the steps of:parametrising the magnetic field produced by the interesting sourcesdisposed in volume V1 in volume V2 as a sum of elementary fields, eachof which is irrotational, sourceless and finite outside volume V1 sothat a presentation of the desired accuracy is achieved for theparametrised magnetic field in volume V2; parametrising the sum magneticfield produced by the interference sources and compensating actuatorsdisposed in volume V3 in volume V2 as a sum of elementary fields, eachof which is irrotational, sourceless and finite outside volume V3 sothat a presentation of the desired accuracy is achieved for theparametrised magnetic field in volume V2; calculating the signal vectorof the measuring device corresponding to each elementary field;measuring the magnetic signal using sensors; and separating the fieldsproduced from sources disposed in different volumes by calculating thecomponents of the measured signal vector in the basis formed by thesignal vectors associated with the elementary fields.
 7. The method asdefined in claim 1, characterised in that the actuator is a coil.
 8. Themethod as defined in claim 2, characterised in that the method furthercomprises the step of: feeding back the measured signal to the actuatorvia an amplifier and a feedback resistor.
 9. The method as defined inclaim 8, characterised in that in the method, the operation of theinterference compensation to be produced using actuators is optimised byvarying the values of the feedback resistors, by increasing the numberof actuators and by varying the locations of the actuators.
 10. Themethod as defined in any one of previous claims 1-3, characterised inthat the method further comprises the step of: placing the assembly ofsensors and the actuators within a magnetically shielding room.
 11. Asystem for shielding an assembly of sensors measuring biomagneticsignals from external interferences, the system comprising: an assemblyof sensors (10, 45) comprising at least one magnetic sensor; a feedbackcoil (14) coupled to each sensors; control electronics (11) controllingthe measuring device; characterised in that the system furthercomprises: at least one magnetic feedback coupled to an assembly ofsensors, wherein the difference signal of one feedback is obtained fromat least one sensor (10, 45) of the aforementioned assembly of sensors;at least one actuator (12, 13, 42, 43) disposed outside the assembly ofsensors for producing a magnetic field in the region of the assembly ofsensors (10, 45), the magnetic field compensating for externalinterferences; and the difference signal that produces a compensationvoltage in the actuator is a linear combination of the signals of one ormore sensors, (10, 45); and control electronics (11) for separating thebiomagnetic useful signal from, the signals (originating from outsidethe measurement region), which signals are produced by the actuators andthe external interferences.
 12. The system as defined in claim 11,characterised in that the system further comprises: generation means(46, 47) of the difference signal for generating the difference signalas a linear combination from the measurement signals of two or moresensors (45) of the assembly of sensors; and the aforementioned magneticfeedback (48, 40) for feeding back the difference signal to theactuators (42, 43), the compensating magnetic field being formed as alinear combination from the magnetic field produced by at least oneactuator (42, 43).
 13. The system as defined in claim 12, characterisedin that the system further comprises: the aforementioned differencesignals and actuators (12, 13, 42, 43) so selected that as the feedbackis switched on, the external interference is minimised.
 14. The systemas defined in claim 11, characterised in that the assembly of sensors(10, 45), the feedback coils (14) and the control electronics (11)function as parts of a magnetoencephalographic device (MEG).
 15. Thesystem as defined in claim 14, characterised in that the system furthercomprises: each measurement sensor (10, 45) coupled to two feedbackloops, the first of them being a channel of an MEG device operating in aflow-locked state and the second a feedback loop controlling thecompensating actuator (12, 13, 42, 43).
 16. The system as ‘defined in ’claim 11, wherein in the SSS method, a magnetic field that has beenregistered using a multi-channel measuring device is analysed in ageometry in which the interesting source (15) is disposed in measurementvolume V1, the sensors (10, 45) measuring the field or the componentsthereof outside volume V1 in volume V2, and the sources of the magneticinterferences and the actuators outside volume V3 and V2 in volume V3,which can be infinite, characterised in that the control electronics(11) is arranged to: parametrise the magnetic field produced by theinteresting sources disposed in volume V1 in volume V2 as a sum ofelementary fields, each of which is irrotational, sourceless and finiteoutside volume V1 so that a presentation of the desired accuracy isachieved for the parametrised magnetic field in volume V2; parametrisethe sum magnetic field produced by the interference sources andcompensating actuators (12, 13, 42, 43) disposed in volume V3 in volumeV2 as a sum of elementary fields, each of which is irrotational,sourceless and finite outside volume V3 so that a presentation of thedesired accuracy is achieved for the parametrised magnetic field involume V2; calculate the signal vector of the measuring devicecorresponding to each elementary field; measure the magnetic signalusing sensors (10, 45); and separate the fields produced from sourcesdisposed in different volumes by calculating the components of themeasured signal vector in the basis formed by the signal vectorsassociated with the elementary fields.
 17. The system as defined inclaim 11, characterised in that the aforementioned actuator (12, 13, 42,43) that produces the magnetic field is a coil that has been connectedto a device containing an assembly of sensors (10, 45) or to a separateframe around the assembly of sensors (10, 45), or to the walls, floor orceiling.
 18. The system as defined in claim 12, characterised in thatthe system further comprises: an amplifier (24), a feedback resistor(25) and an actuator (12) as parts of the magnetic feedback.
 19. Thesystem as defined in claim 18, characterised in that in the system, theoperation of the interference compensation to be produced usingactuators (12, 13, 42, 43) is optimised by varying the values of thefeedback resistors (25), by increasing the number of actuators (12, 13,42, 43) and by varying the locations of the actuators (12, 13, 42, 43).20. The system as defined in claim 11, characterised in that the systemfurther comprises: a magnetically shielding room as a place for theassembly of sensors (10, 45) and the actuators (12, 13, 42, 43).